With increasing population and urbanisation together with improved standards of living, the treatment of water as well as of sewage and like waste waters is of cardinal importance and indeed of great public concern, not only in the preservation and improvement of the environment, but also in the conservation and recycling of water of sufficient purity for industrial and domestic uses, particularly for drinking and food preparation.
The treatment of sewage comprises the stages of settlement to remove settleable suspended matter followed by biological treatment of the supernatant liquid whereby dissolved organic material is converted into settleable biomass, and by subsequent removal of the biomass by further settlement.
In the biological stage of the treatment, the water containing dissolved organic matter is contacted with air in the presence of aerobic bacteria. The bacteria are thus cultured and grow to form with the dissolved organic matter a biomass together with carbon dioxide and water.
Contact may take place in aerated tanks (activated sludge process). Alternatively, biofilms containing the aerobic bacteria supported on and adhering to solid media may be employed, over which the supernatant liquid is sprayed or otherwise distributed. Such solid material may be in the form of a percolating filter, also called a trickling or biological filter.
Filtration installations of the latter kind are extensive in area and subject to various technical disadvantages and a more sophisticated contacting arrangement is known as the rotating biological contactor (RBC) in which parallel discs with attached biofilms are rotated alternately through the supernatant liquid to be treated and the air, thus facilitating oxygen transfer into the biofilm.